boot/zephyr/prj.conf: remove SOC_FLASH_STM32F4, make it generic
boot/zephyr/prj.conf: remove SOC_FLASH_STM32F4, make it generic

Make it generic and let the SoC itself handle the FLASH driver support
as part of the Zephyr build system.

Signed-off-by: Ricardo Salveti <6fd42f733b6586c555e01d4ff3116fc7e73e7330@linaro.org>

utzig/mcuboot,tamban01/mcuboot,utzig/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,utzig/mcuboot,runtimeco/mcuboot,utzig/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,tamban01/mcuboot,utzig/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot